The SAMI Galaxy Survey: Physical drivers of stellar-gas kinematic misalignments in the nearby Universe

TL;DR

This study analyzes the physical causes and characteristics of stellar-gas kinematic misalignments in nearby galaxies, revealing their dependence on galaxy morphology, star formation, and gas accretion processes.

Contribution

It provides a comprehensive analysis of the physical drivers behind stellar-gas misalignments using a large galaxy sample from the SAMI survey, highlighting the roles of accretion and outflows.

Findings

12% of galaxies show stellar-gas misalignments.

Misalignments are more common in early-type/passive galaxies.

Accretion is the dominant cause of misalignments, with some due to outflows.

Abstract

Misalignments between the rotation axis of stars and gas are an indication of external processes shaping galaxies throughout their evolution. Using observations of 3068 galaxies from the SAMI Galaxy Survey, we compute global kinematic position angles for 1445 objects with reliable kinematics and identify 169 (12%) galaxies which show stellar-gas misalignments. Kinematically decoupled features are more prevalent in early-type/passive galaxies compared to late-type/star-forming systems. Star formation is the main source of gas ionisation in only 22% of misaligned galaxies; 17% are Seyfert objects, while 61% show Low-Ionisation Nuclear Emission-line Region features. We identify the most probable physical cause of the kinematic decoupling and find that, while accretion-driven cases are dominant, for up to 8% of our sample, the misalignment may be tracing outflowing gas. When considering only misalignments driven by accretion, the acquired gas is feeding active star formation in only $\sim$1/4 of cases. As a population, misaligned galaxies have higher S\'ersic indices and lower stellar spin & specific star formation rates than appropriately matched samples of aligned systems. These results suggest that both morphology and star formation/gas content are significantly correlated with the prevalence and timescales of misalignments. Specifically, torques on misaligned gas discs are smaller for more centrally concentrated galaxies, while the newly accreted gas feels lower viscous drag forces in more gas-poor objects. Marginal evidence of star formation not being correlated with misalignment likelihood for late-type galaxies suggests that such morphologies in the nearby Universe might be the result of preferentially aligned accretion at higher redshifts.